1. Field of the Invention
The present invention relates generally to a liquid fuel rocket engine, and more specifically to monitoring for two-phase flow in a rocket engine feed line.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A liquid fuel rocket engine burns a fuel with an oxidizer in a combustion chamber of a rocket nozzle to produce thrust. The fuel (such as liquid hydrogen) is stored in a fuel tank while the oxidizer (such as liquid oxygen) is stored in an oxidizer tank. Both tanks are typically pressurized to force the liquid from the tank into an entrance to a turbopump that then increase the pressure for delivery to the combustion chamber. The fuel is delivered into the fuel turbopump and the oxidizer is delivered into the oxidizer turbopump.
The fuel and oxidizer reservoir tanks are pressurized in order to deliver the liquids into the inlet of an inducer. The inducer is a low pressure (relative for rocket engines) pump that increases the pressure of the liquid (LOX or H2) for delivery into the centrifugal pump that produces the high pressure for discharge into the combustion chamber. The higher the reservoir pressure, the less likely that cavitation will occur within the inducer. However, the higher the reservoir tank pressure the thicker the tank walls must be in order to withstand the higher pressures. Thicker reservoir walls results in heavier reservoir tank and thus heavier vehicle weight.
Optimal performance in rocket engine turbo-pumps depends upon the condition of the inlet propellant flow, in terms of both a uniform velocity profile and the quality of the fluid. Local bubbles in rocket engine pump feed lines serve as the inception sites for cavitation in the pump. These bubbles further grow into vapor cavities as the flow gains velocity just downstream of the inducer leading edge, where the local pressure can drop below vapor pressure. The further the onset of cavitation can be delayed, the higher the operating suction specific speed of a pump. Thus knowledge of the upstream flow quality is critical to the overall health and performance of the engine.
Current state-of-the-art technologies to measure two-phase flow include optical sensors or capacitive sensors. Capacitive sensors rely on the difference in dielectric constant between the liquid and vapor. Optical sensors depend on the difference in index of refraction between liquid and vapor.
Disadvantages of capacitive sensors include sensitivity to void fraction distribution (flow regime, whether “bubbly” or “slug”), changes in electrical properties of the fluid with respect to temperature, and presence of local electromagnetic field all affect the accuracy of the measurement. These sources of uncertainty have been calculated to be on the order of +/−6% in a laboratory test section. The presence of local electromagnetic field from other equipment also requires substantial sensor shielding.
The accuracy of optical sensors depends on knowledge of the net liquid flow rate. If this value is known typical uncertainties are +0/−6%, otherwise uncertainties as high as +0/−16% have been reported. Another major limitation of optical sensors is the size of the bubbles, with diminishing bubble size causing a large underestimation (˜50% under) of the vapor concentration. This inaccuracy is particularly evident in the bubble flow regime, where tiny bubbles are interspersed throughout the fluid. The limitations of point source fiber optic sensors can be overcome by employing a “plane of light”.
Ultrasonic flow meters represent current state-of-the-art technology which uses sound to measure flow. However, the coldest temperature demonstrated for cryogenic fluid flow is around −200 degrees Celsius, cold enough for LOX but not LH2. Ultrasonic flow meters have many sources of uncertainty, including fluid sound speed, flow profile, and “installation” sources of uncertainty (such as pipe wall, lining, roughness, and cross talk). One of their major sources of uncertainty is two-phase flow. The presence of vapor bubbles affects the way sound waves travel through the fluid medium.
An existing embodiment of a cryogenic two-phase flow meter also uses an acoustic signal to determine flow velocity. While capacitive sensors are essentially thin strips of two conductive materials and optical fiber sensors can be manufactured very small (125 μm diameter), the acousto-optic sensor appears to require a large amount of additional equipment, which translates to added weight on the launch vehicle.